An in-flight investigation of the effect of head up display (HUD) symbol control laws has been conducted using a variable stability T-33 aircraft. The results indicate that HUD delays (pure time delays) of the order to of 140 msec in up-and-away flight have little effect on flying qualities. Sampling at intervals of 100 msec (10 hz) has a marked effect and degrades Cooper-Harper pilot ratings from a rating of 2 (good with negligible deficiencies) to a rating of 6 (very objectionable but with tolerable deficiencies). For the power approach case, the delays of up to 230 msec have negligible effect. In the power approach case, sampling at 10 hz had no effect. Keywords: Symbology, Instruments, Flight control systems, Flight tests, Display systems.